A typical refrigerant based heating or cooling system, such as an air conditioning or heat pump, generally includes several principal mechanical components. These components include a compressor, a condenser coil and fan and an evaporator coil and fan. A chemical refrigerant is the working fluid which is designed to circulate in a closed loop within the system. In the case of a heat pump, the system may include one or more reversing valves to permit switching between heating and cooling modes.
Maintenance of an appropriate level of refrigerant charge in refrigerant based heating or cooling systems is important for efficient operation of the system. If the refrigerant charge level in the equipment is too low, such as due to a refrigerant leak, large reductions in system capacity and efficiency may result, up to 30% or more being possible. In addition, knowledge of the refrigerant level permits diagnosing leaks and other system problems.
Refrigerant leakage can cause many problems. First, the release of some refrigerants into the environment are believed to result in environmental damage, such as an increase in the greenhouse effect. Second, when the refrigerant charge becomes insufficient, the reliability and cooling performance of the system suffers. Thus, a refrigerant based system having a low charge is inefficient. Thus, there is significant interest in developing systems and methods for detecting low refrigerant charges.
Unfortunately, conventional approaches for determining proper refrigerant charge may require complex valve arrangements and other components, may be cumbersome, and may be intrusive to the refrigerant system. These approaches may also be unreliable and inaccurate.
Moreover, some approaches are designed for exclusive use by service professionals. For example, one approach for determining proper refrigerant charge in a heat pump is disclosed in U.S. Pat. No. 3,153,913 to Brody entitled “Refrigeration System Including Charge Checking Means”. Brody discloses a sight glass in a refrigerant charge container which is connected between two heat exchangers of a heat pump. A portion of the refrigerant charge is stored in the container during the heating cycle, while the container also has a total capacity capable of containing substantially the entire optimum liquid refrigerant charge. The heat pump requires control means for controlling the pressure differential between the two heat exchangers, with a smaller circulating charge being desired on the heating cycle than on the cooling cycle. The container is connected between the indoor heat exchanger and flow restricting means. The container provides means for obtaining a difference in the effective or circulating charge of refrigerant on the heating and cooling cycles of operation of the heat pump.
To check the charge, the heat pump is operated in the cooling mode and a normally open valve provided in the line connecting the indoor heat exchanger with the container is closed, while a normally closed valve in the charge checking conduit is opened. Closing the valve prevents flow of liquid refrigerant to the indoor heat exchanger. The liquid refrigerant flowing into the container can no longer flow into the indoor heat exchanger, while any liquid refrigerant contained in the indoor heat exchanger will be evaporated and returned to the compressor through the low pressure conduits.
The refrigerant condensed in the indoor heat exchanger during the charge checking cycle flows through a capillary into the container where it is maintained in a liquid state by pressure. During the charge checking, any liquid refrigerant stored in any lower pressure portion of the system is transferred to the container. In addition, the sight glass is positioned in an upper portion of the container at the location of the desired liquid/gas interface level. Because it may be difficult to observe the liquid/gas interface with positive accuracy, another valve may be closed to prevent reverse flow of refrigerant from the container and the compressor so that a static reading is obtained. After the proper amount of charge is determined, the various valves are returned to their normal operating positions. In addition, the flow restrictor substantially reduces the refrigerant flow through the system during the charge measuring cycle relative to the flow under ordinary operation.
Brody requires operation in the cooling mode, a special container, manipulation of several valves, and other complexities to obtain an indication of a desired liquid/gas interface level relating to the refrigerant charge. The apparatus is complex and the procedure is cumbersome.
More generally, for systems which use a sight glass, the degree of under-charge must be severe to generally produce bubbles in the liquid line. Accordingly, when bubbles in the liquid line are used to detect under-charge of refrigerant, the system usually has been operated in an under-charged condition for a fairly lengthy period of time.
U.S. Pat. No. 4,114,448 to Merritt (Merritt) entitled “Service Apparatus” discloses an apparatus for servicing refrigeration systems which avoids breaking into a refrigeration line for the purpose of measuring refrigerant pressures. An added feature is the ability of the apparatus to provide either absolute temperature read-outs for the temperature at desired locations, or a differential temperature read-out for the temperatures between two desired locations. Moreover, the disclosed apparatus permits vacuum read-outs and pressure read-outs, again without the necessity of breaking into the refrigeration lines.
Merritt has limited utility as it is designed as a service tool. Usually by the time a service person is called, most of the refrigerant in the system is probably gone already. Merritt does not alert an owner of a developing refrigerant leak. In addition, Merritt uses the ideal gas law (PV=nRT) to relate refrigerant pressure to the measured refrigerant temperature. Thus, his measurements are performed in system locations where the refrigerant is in the vapor or gas form, rather than in the two-phase region, because the ideal gas law does not apply in liquid-vapor two-phase regions.
Other conventional approaches for determining proper refrigerant charge include a complete purge and measured refill of refrigerant for the apparatus, or the commonplace method of attaching gauges, taking pressure readings, obtaining indoor and outdoor temperatures, and applying the data so obtained to the equipment manufacturer's charging chart. These methods are generally all very cumbersome and time-consuming. Thus, conventional approaches for determining proper refrigerant charge have significant shortcomings.